1. Field of the Invention
This invention relates generally to an optical modulator for an analog optical link and, more particularly, to an optical phase modulator for an analog optical link that includes long velocity-matched electrodes having pre-emphasis to provide an RF loss versus RF frequency needed to produce the desired Vxcfx80 versus RF frequency over a wide bandwidth.
2. Discussion of the Related Art
Analog optical links are used in various optical communications systems where the transmission of large bandwidth signals are required, without the need for analog-to-digital (A/D) converters or digital-to-analog (D/A) converters. Digitization of analog RF signals is an excellent way of enabling information transfer, and the capability of digital optical links to convey such information is well known. However, digitizing an analog RF signal having a 5 GHz bandwidth requires approximately a 100 Gbps digital link throughput, which is well beyond current data link capabilities. Therefore, analog links are required to meet the desired wideband transmission requirements.
Analog optical links transmit RF signals modulated onto an optical carrier signal. The optical carrier signal generally is transmitted along a fiber optic cable or through free space to a receiver where it is demodulated to recover the RF data. The optical link allows the RF data to be transmitted with low losses and at high bandwidths, and thus is attractive in many communications systems to provide the desired performance, especially high frequency RF communications systems that transmit signals in the GHz bandwidth range. Also, telescopes used to transmit optical signals in free space have a much greater directivity than RF antennas of comparable size.
The analog optical links being discussed herein need to be wideband analog signals having high-fidelity. A wideband signal discussed herein may be up to tens of GHz. By high fidelity, it is meant transmission of signal information with resultant dynamic range and a signal-to-noise (SNR) equivalent to that achievable if the signal were digitized with more than six bits and transmitted using a digital communication link. To have the desired performance for various communications systems, the optical link must provide a good dynamic range, i.e., allow the simultaneous transmission of signals having widely varying amplitudes that do not interfere with each other, with minimal optical power requirements.
Currently, intensity modulation (IM) is the dominant optical modulation choice for analog optical links. In IM, the intensity of the optical light is modulated with the RF signal. Unfortunately, IM does not provide high enough performance because significant transmission power is required to provide the desirable dynamic range and signal-to-noise ratio (SNR) for a particular application. In fact, ideal linear IM requires 9 dB more received optical power than ideal suppressed carrier amplitude modulation (AM) to get the same demodulated SNR. To overcome this problem, known intensity modulation optical links provide a series of optical amplifiers to boost the optical carrier signal power as it propagates along an optical fiber. The number of optical amplifiers needed can be costly. Also this technique cannot be used for long distance free space links.
Wideband frequency modulated (FM) or phase modulated (PM) optical links can theoretically use the extremely wide bandwidth available at optical frequencies to achieve much better dynamic range and SNR than IM optical links for the same received power. For example, phase modulation having a peak phase deviation of 10 radians has a 26 dB greater link SNR potential compared to ideal IM, and a 17 dB greater SNR potential than suppressed-carrier AM.
Known FM or PM communications systems must significantly modulate the carrier frequency or phase to achieve better dynamic range and SNR performance than AM. In other words, the frequency deviation or phase deviation of the carrier signal which is induced by the RF input signal must be large enough to increase the bandwidth of the modulated carrier substantially beyond that of an AM modulated carrier.
Phase modulated optical links generate an additive noise floor that is higher at lower frequencies. Therefore, the sensitivity of multi-octave frequency ranges is degraded at the lower frequencies. Also, since optical frequency demodulators are generally used to demodulate a phase modulated signal, an RF integrator is needed to recover the signal. Multi-octave RF integrators generally have a large gain slope across the signal frequency range and can be difficult to implement.
Frequency modulated optical links do not have the low frequency noise problem that phase modulated optical links have. However, direct wideband frequency modulation of an optical beam is much more difficult and less desirable than external phase modulation. Direct frequency modulation of the carrier wave laser source requires the laser beam to be co-located with the RF input signal so that no photonic remoting of the laser beam is allowed. Direct frequency modulation can also interfere with line width reduction circuitry, which is important to maintain the low overall FM or PM link noise floor. External frequency modulation using an RF integrator followed by an external phase modulator has the same gain slope and implementation problems associated with the phase demodulator.
What is needed is an optical modulator for a wideband, high fidelity optical link that acts like a phase modulator at high frequencies and a frequency modulator at low frequencies. It is therefore an object of the present invention to provide such an optical modulator.
In accordance with the teachings of the present invention, an optical modulator for an optical link is disclosed that operates as a phase modulator at high frequencies and as a frequency modulator at low frequencies to provide a suitable signal-to-noise ratio over a wide frequency band. The modulator includes an optical waveguide and opposing RF electrodes formed in the waveguide. An RF input signal is applied to the electrodes to create an electric field that changes the index of refraction of the waveguide to affect the propagation speed of an optical carrier signal, and thus provide the modulation.
The electrodes are long velocity-matched electrodes that provide the RF loss versus RF frequency needed to produce the desired Vxcfx80 versus RF frequency over the entire bandwidth. The lower Vxcfx80 at lower RF frequencies will emphasize the lower frequencies and improve the low-frequency fidelity of the optical link by boosting the low-frequency signals above the excess link noise generated in the demodulator. If the electrode losses are proportional to the RF frequency and the waveguide is long enough, then the magnitude response of the link is like a frequency modulated link. If the electrode losses are proportional to the square root of the RF frequency and the waveguide is long enough, than the magnitude response of the link is between frequency modulation and phase modulation. At higher frequencies, the modulator operates as a true phase modulator.
Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings.